For efficient power generation of a fuel cell, it is important to maintain an electrolyte membrane in a suitable wet state. Specifically, if the electrolyte membrane is too wet, flooding occurs or a purge operation at stoppage is necessary in preparation for sub-zero start-up. Further, if the electrolyte membrane is insufficiently wet, a voltage of a fuel cell stack may drop and an output may be largely reduced. Accordingly, in JP2007-115488A issued by the Japan Patent Office in 2007, a pressure regulating valve and a cathode compressor are controlled to set such cathode gas pressure and cathode gas flow rate that an electrolyte membrane is maintained in a suitable wet state. Particularly in the case of controlling to make the electrolyte membrane wetter in consideration of fuel economy, a rotation speed is first reduced to lower power consumption of the cathode compressor and then the pressure regulating valve is closed to increase the pressure.